It's a problem with BOTH companies.
1. Sony's idiotic PS3 design splits its already pitifully low memory into two dedicated 256mb segments.
2. Bethesda's games are stupidly massive open worlds with tons of crap to interact with, meaning tons of crap that gets moved and has to be stored into its memory.
Normally, these two separate things are not a problem in of themselves, but, because of Sony's idiotic PS3 memory split design, and Bethesda's massive open worlds needing to store tons of data, Skyrim ends up quickly maxing out the PS3's stupidly low total of available memory. Had Sony not used such an idiotic design, the game would run fine. Which is why the game runs fine on the Xbox, because it's memory isn't split, and thus, Skyrim can't max it out.
HOWEVER, Bethesda SHOULD have been aware of this, and taken steps to rectify it BEFORE the game was released, but they they didn't, and they are idiots for not doing so and inconveniencing the customer.
Its a combination of Sony's dumb design move, and Bethesda not taking steps to work around the problem as much as they could from the get-go.
We can only pray the PS4 doesn't do the same thing.